US7191503B2 - Method of manufacturing a piezoelectric actuator - Google Patents

Abstract

A method for making a piezoelectric actuator comprises coating at least one of a first surface and a second surface of a piezoelectric element with a polyimide adhesive. The piezoelectric element is then heated to dry the adhesive. Afterwards, the piezoelectric element is inserted between a first metallic layer and a second metallic layer to form an assembly. The assembly is placed in a press. While the assembly is in the press, the polyimide adhesive is cured at a curing temperature which does not depole the piezoelectric element, thereby bonding the piezoelectric element between the first metallic layer and the second metallic layer.

Description

This application is a national stage of international application PCT/US01/28947 filed 14 Sep. 2001 which designates the U.S., and which claims the benefit and priority of U.S. Provisional Application 60/233,248, filed Sep.18, 2000, all of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention is in the field of the manufacture of ferroelectric actuators and miniature diaphragm pumps using these actuators as the prime mover. In the best mode the actuators are piezoelectric.

BACKGROUND ART

The prior art for this invention may be grouped as follows:

• I. U.S. Pat. Nos. 5,471,721, 5,632,841, 5,849,125, 6,162,313, 6,042,345, 6,060,811, and 6,071,087 showing either prestressing of piezoelectric actuators, or dome-shaped piezoelectric actuators, or both. This prior art is generally in apposite because the present invention does not use a prestressed or dome-shaped piezoelectric actuator.
• II. U.S. Pat. Nos. 6,179,584, 6,213,735, 5,271,724, 5,759,015, 5,876,187, 6,227,809 showing so-called micropumps. Such pumps generally pump only a drop of fluid at a time; because of the small forces and low Reynolds numbers involved, this prior art is generally in apposite.
• III. U.S. Pat. Nos. 4,034,780, 4,095,615 showing flapper valves. These are flappers mounted on a separate hinge. No prior art was found showing a flex valve with a miniature pump.
• IV. U.S. Pat. Nos. 5,084,345, 4,859,530, 3,936,342, 5,049,421 showing use of polyimide adhesives for various purposes, including bonding metals and other materials to film.
• V. U.S. Pat. Nos. 4,939,405, 5,945,768 showing electrical driver circuits for piezoelectric actuators.
• VI. U.S. Pat. Nos. 6,227,824, 6,033,191, 6,109,889, German WO 87/07218 showing various kinds of pumps incorporating piezoelectric actuators.

DISCLOSURE OF INVENTION

This invention is a method for making a high-displacement ferroelectric actuator, in this case a piezoelectric actuator. This piezoelectric actuator may then be used as the diaphragm in a small diaphragm pump. The pump is small, lightweight, quiet, and efficient. The best mode, a round pump about 40 mm [1.5″] in] diameter by about 13 mm [0.5″] thick and weighing approximately 35 g [one ounce], can pump upwards of 450 milliliters of water or other fluids per minute. These pumping rates are accomplished using a six-volt battery at 25 ma driving through a small electronic driver circuit, approximately 25 mm [1″] square. This circuit forms part of the invention. The one way valve[s] necessary for operation of the invention are flex valves in which a thin film of polyimide acts as the working element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the pump of the invention with the parts in the positions they would be for the best mode.

FIG. 2 is a sectional view of the pump alongline2—2 ofFIG. 1.

FIG. 3 is a sectional view of the press used to make the piezoelectric actuators of the invention.

FIG. 4 shows the driver circuit for the piezoelectric actuator used with the pump.

FIG. 5 is a partially diagrammatic view showing an alternative embodiment of the invention in which the pump chamber is reduced in size.

FIG. 6 is a partially diagrammatic view showing another alternative embodiment of a pump in which the inlet and outlet are perpendicular to the plane of the actuator.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows how the piezoelectric actuator of the present invention may be used in a miniature diaphragm pump. Thepump10 is generally in the form of a circular short cylinder. It includes thepump body12,piezoelectric actuator14,pump cover16 and piezoelectric actuatorelectronic driver circuit18. Thepump body12 haslugs20 for mounting the pump to any substrate.Inlet22 andoutlet24 are part of thepump body12 though they could be separate pieces otherwise fastened to the pump body. Thepump cover16 is essentially the same diameter and of the same material as thepump body12. The material would ordinarily be of a standard plastic such as acetal [DELRIN®], PVC, or PC, or of a metal such as stainless steel or brass. These are preferable since they can be easily machined or thermally formed. Thecover16 may be fastened to thepump body12 by any means such as by a fast-curing adhesive while thepump body12 andcover16 are under compression such as by clamping. The pump cover has anopening26 for venting the space above theactuator14.

The dimensions of the pump depend on the particular application. In the best mode thepump body12 is about 40 mm [1.5″] in diameter. Apump chamber30 is formed in the center of thepump body12, for example by molding or machining. Thepump chamber30 is about 28 mm [1.125″] in diameter or about 3 mm [⅛″] less in diameter than the diameter of thepiezoelectric actuator14. Thechamber30 is about 6 mm [0.25″] deep. Aseat32 about 3 mm [0.125″] wide and about 2 mm [0.070″] deep is provided in thepump body12 at the top of thepump chamber30. As shown inFIG. 2 thepiezoelectric actuator14 is mounted on theseat32 to form the diaphragm in the top of thepump chamber30.

To assemble the pump a sealingwasher34 the same diameter as the piezoelectric actuator is put on theseat32 to seal the pump chamber when thepiezoelectric actuator14 is put in place. The sealingwasher34 may be of a relatively soft material such as Buna-N or silicon rubber to account for any irregularities in the mating surfaces and ensure a good seal between the actuator14 and thepump body12. Once thepiezoelectric actuator14 is in place an o-ring seal36 is placed on top of thepiezoelectric actuator14 to hold thepiezoelectric actuator14 in place and seal it from thecover16. Thecover16 of the same outside diameter as thepump body12 base but only about ⅛″ thick is then put in place. Sealingwasher34 and o-ring seal36 are referred to collectively as the pump seals, even though they both have the additional function of fixing theactuator14 in place with respect to thepump body12. Thecover16 is then fastened to thebody12 while under compression, for example by adhesive under clamping pressure, to seal thepiezoelectric actuator14 to thebody12 and fix theactuator14 in place to allow pumping action.

The process for making thepiezoelectric actuator14 generally is as follows:

Apiezoelectric wafer38 formed of a polycrystalline ferroelectric material such as PZT5A available from Morgan Electro Ceramics is obtained. As the name implies this material is actually a ceramic. It is processed into the highdisplacement piezoelectric actuator14 by laminating thepiezoelectric wafer38 between ametal substrate layer40 and anouter metal layer42 as shown in FIG.2., where the thicknesses of the three layers and the adhesive between them are exaggerated for clarity. Thebonding agent41 between thelayers38 and40 is a polyimide adhesive. This lamination process does several things: It ruggedizes thepiezoelectric actuator14 because the metal layers keep the piezoelectric from fracturing during high displacement. It permits higher voltage due to the relatively low dielectric constant of the polyimide adhesive, thereby allowing 3–5 times higher displacement than a conventional piezoelectric. Being laminated between metal layers using a high performance polyimide adhesive makes the piezoelectric actuator highly resistant to shock and vibrations. With this invention piezoelectric actuator devices can be used in environments as hot as a continuous 200° C., compared to only 115° C. for a conventional piezoelectric. The significant increase in temperature is due to the polyimide adhesive used in the bonding process which is unaffected by temperatures up to 200° C. Epoxy adhesives used in conventional piezoelectrics normally can withstand temperatures up to only 115° C. This increase in operating temperature would allow the pumps of this invention to be used in a variety of pump applications, even pumping boiling water continuously.

Thepiezoelectric wafers38 are available from the vendor mentioned in various shapes and thicknesses. For the invention circular wafers 25 mm [1″] in diameter and 0.2 mm [0.008″] thick were found to be optimum. Square wafers were tried but did not give maximum displacement. In general the thinner the wafer, the greater the displacement at a given voltage, but the lower the force. The 0.2 mm [8-mil] thickness gives the best flow rate for the diameter of the wafer.

In the best mode stainless steel 0.1 mm [0.004″] thick is used for thesubstrate layer40, the layer in contact with the pumped liquid. Stainless steel is chosen for its compatibility with many liquids, including water, its fatigue resistance, its electrical conductivity and its ready availability at low cost. Aluminum 0.05 mm [0.001″] thick is used for theouter layer42 primarily for its electrical conductivity in transmitting the actuating voltage to thepiezoelectric wafer38 across its surface, but also for its robustness and ready availability at low cost.

The diameter of thepiezoelectric wafer38 being about 25 mm [1″] as noted above, the diameter of thesubstrate layer40 is about 40 mm [1.25″]. The setback of thewafer38 from the edge of thesubstrate layer40 is an important feature of the invention. This leaves a rim that serves as a clamping surface for the actuator assembly. This means that the entirepiezoelectric wafer38 is free and relatively unconstrained, except insofar as it is bonded to thesubstrate40 and theouter layer42. This allows maximum displacement of theactuator14, ensuring maximum flow of liquid through the pump.

The diameter of theouter layer42 is smaller than the diameter of thewafer38. This setback of theouter layer42 from the edge of thewafer38 is done to prevent arcing over of the driving voltage from theouter layer42 to thesubstrate layer40.

Other materials and thicknesses may be used for the enclosing layers40 and42 as long as they meet the requirements noted.

Of special note is that the piezoelectric actuator of the invention is flat. In much of the prior art the actuator is dome-shaped, it being supposed that this shape is necessary for maximum displacement of the actuator and therefore maximum capacity of the pump for a given size actuator. Special molds and methods are proliferated to produce the shapes of the actuator considered necessary, or to produce a prestress in the actuator that is supposed to increase its displacement. Our tests of the invention have shown, however, that a dome shape is not necessary, and that the flat actuator has a higher pumping capacity for a given size than any known pump in the prior art. As such the actuator is much simpler to produce in large quantities, as the following will demonstrate. The flat shape also means that the pump may be smaller for a given application. A flat actuator is also inherently easier to mount in any given application than a dome shaped actuator would be. Furthermore, pumps using the actuator have been shown to have sufficiently long life for numerous applications. Many manufacturers whose names are household words are using or testing this invention.

The process for making thepiezoelectric actuator14 specifically is as follows:

• 1. Thepiezoelectric wafer38 and and enclosinglayers40 and42 are cleaned using a solvent that does not leave a residue, such as ethanol or acetone. All oil, grease, dust and fingerprints must be removed to ensure a good bond.
• 2. Thepiezoelectric wafer38 is then coated on both sides with athin layer41, not more than 0.1 mm [0.005″], of a high performance polyimide gel adhesive such as that available from Ranbar Inc. The gel should contain a minimum of 25% solids to allow sufficient material for a good bond after the solvent is driven off
• 3. Thepiezoelectric wafer38 is then placed under a standard heat lamp for about 5 minutes to remove most of the solvent from the gel and start the polyimide gel polymerization process. Both sides of the piezoelectric must be cured under the heat lamp since both sides are to be bonded to metal.
• 4. Once the adhesive is dry to the touch, thepiezoelectric wafer38 is then placed between thesubstrate layer40 and theouter layer42.
• 5. The assembly is placed in a special press. This press was developed specifically for makingpiezoelectric actuators14 and provides uniform temperature and pressure to ensure a good bond between the three components of the actuator. Referring to the best mode shown inFIG. 3 the press comprises two 300 mm [12″] square by 6 mm [¼″] thick plates ofaluminum101 held together withthumbscrews102, four on each edge. To ensure uniform pressure while in the press, thebottom plate101 of the press is covered with a sheet of lowcost polyimide film104 such as Upilex available from Ube Industries Ltd. Thepiezoelectric actuators38 are placed on the film and a sheet of high temperature, 4 mm [⅛″]thick rubber106 is placed over the piezoelectric actuators. The rubber on top and the film on bottom cushion thepiezoelectric actuators38 providing even distribution of pressure when the press is taken to temperature. Of course other dimensions of the press plates are possible.
• 6. Once the piezoelectric actuators are placed in the press thethumbscrews102 are made finger tight.
• 7. The press is then placed in a standard convection oven for thirty minutes at about 200° C.
• 8. The press is removed from the oven, allowed to cool to a safe temperature, and theactuators14 removed from the press.

Thepress100 is the result of an effort to develop a low cost, rapid process for manufacturing piezoelectric actuators. The press takes advantage of the thermal expansion of thealuminum plates101 which creates the necessary pressure to cause the polyimide adhesive to bond to thepiezoelectric wafer38 andmetal layers40,42 while it is at curing temperature. The press can be put into the oven, and taken out, while the oven is at temperature thereby allowing continuous operation during the manufacturing process. The abrupt change in temperature does not affect thepiezoelectric actuators14 since they will remain under pressure even while the press is removed from the oven and allowed to assume room temperature.

Of special note is that this press process is one of further driving off the solvent and curing the polyimide at a relatively low temperature. Prior art processes for making similar piezoelectric actuators require the mold/press to be taken to much higher temperatures, high enough to melt the polyimide adhesive. Furthermore, since such high temperatures depole the piezoelectric ceramic, it is necessary to pole it again at the end of the process. The present invention eliminates this step altogether, thus contributing to the lower cost of manufacturing the piezoelectric actuators.

Using these simple methods and hardware it is possible to manufacture hundreds of thousands ofpiezoelectric actuators14 per month, or even more, depending on the scale of the operation desired.

The principle of thepiezoelectric actuator pump10 is the same as for any diaphragm pump. Normally the diaphragm in a diaphragm pump is operated by a cam or a pushrod connected to a motor or engine. This is not the case in thepiezoelectric actuator pump10. Thepiezoelectric actuator14 acts as the diaphragm and moves when a pulsed electric field is imposed across thepiezoelectric wafer38 by means of the enclosing layers40 and42. This varying electric field causes thepiezoelectric actuator14 to expand and contract. As theactuator14 expands, with its edge constrained, it assumes a slight dome shape as the center of the actuator moves away from thepump chamber30. This draws liquid into thepump chamber30 through theinlet22. When thepiezoelectric actuator14 contracts it moves toward the liquid, forcing it out of thepump chamber30 throughoutlet24.

One of the problems with prior art piezoelectric actuators has been the voltage necessary to drive the piezoelectric. To provide power to thepiezoelectric actuator pump10 theelectrical driver18 shown inFIG. 4 was invented that converts the voltage from any six volt d.c. power source to an alternating current of over 200 volts peak-to-peak. This voltage is sufficient in the preferred embodiment to drive a piezoelectric actuator to attain the pumping rates noted above. In the circuit inFIG. 4 point A is connected to thesubstrate layer40 while point B is connected to theouter layer42.

Piezoelectric actuators perform better when the peak-to-peak voltage is not evenly balanced. They respond better to a positive voltage than the same negative voltage. Thus thecircuit18 has been designed to produce alternating current with the voltage offset to 150 volts positive and 50 volts negative. This is sufficient voltage for the piezoelectric actuator to make a very efficient pump. While a sinusoidal wave will work, at the lower frequencies and voltages, a square wave makes the piezoelectric more efficient. Values of the circuit components inFIG. 4 are as follows:

	R1	8 to 20MΩ
	R2
	8 to 20 MΩ
	R3	680KΩ
	R4
	1 MΩ
	C1	0.1 μF
	C2	0.1 μF
	C3	0.1 μF[200 v]
	C4	0.47 μF[200]
	L1	680 μH
	D1	BAS21 diode

U1 is an IMP 528 chip designated an electroluminescent lamp driver. In this circuit, with the other components, it serves to shape the pulses and amplify them to the 200 volt peak-to-peak value needed to drive thepiezoelectric actuator14. The values of R1 and R2 are chosen to vary the frequency of the output between about 35 Hz and about 85 Hz, depending on the particular application.

This circuit is composed of miniaturized components so it may be contained in abox302 approximately 25 mm [1″] square by 6 mm [¼″] deep. It has only eleven off-the-shelf surface mount components. Thebox302 may be mounted anywhere in proximity to thepump10. In the best mode it is mounted on top of the pump, as shown inFIGS. 1 and 2, for example by an appropriate adhesive. Leads15 run from thedriver circuit18 and are fastened to spring loadedcontacts304 such as those sold by the ECT Company under the trademark POGO®. Thesecontacts304 are mounted in abox306 on top ofpump cover16 and project through thepump cover16 to make contact with the twolayers40 and42. This small driver circuit eliminates the need for the large power supplies and transformers used in prior art piezoelectric applications. Alternatively theleads15 could be run through an opening in thecover16 and fastened electrically to thelayers40 and42, as by soldering. O-ring36 is soft enough to accommodate the soldered point on thesubstrate layer42.

Several conventional types of one-way valves were evaluated as inlet and outlet valves for thepiezoelectric actuator pump10. All had various drawbacks including bulk and poor response to the dynamic behavior of thepiezoelectric actuator14. Aninline flex valve200 was invented that is well adapted to the action of thepiezoelectric actuator14 as shown inFIG. 2. The working element of the flex valve is anelliptical disk202 of polyimide film about 0.05 mm [0.002″] thick. Thedisk202 is the same size and shape as the end of a short piece ofrigid tube204 formed at about a 45° angle to the axis of therigid tube204. The inside diameter of therigid tube204 is the same as the inside diameter of theinlet22 oroutlet24 of thepump body12.Rigid tube204 is captured in the end of theflexible system conduit206 which slips over the inlet/outlet22,24 and carries the system liquid, as shown inFIG. 2.Valve disk202 is attached to the nether end of the slanted surface at the point designated203 by any sufficient means such as by adhesive or thermal bonding. Asimilar flex valve200 may be placed in theoutlet24. Bothdisks202 of both valves would point in the same direction downstream. However, it was found in operating thepump10 that it would pump at full capacity with no valve at all in the outlet. It is postulated that the liquid in the inlet circuit, even with the inlet valve partially open, provides enough inertia to act as a closed inlet valve. Operation with only the inlet valve is considered to be the best mode.

Thisflex valve200 is an important feature of the invention. It is of absolute minimum bulk. The mass of thedisk202 is also about as light as it could possibly be so it reacts rapidly to the action of theactuator14. When it is open it presents virtually no resistance to the system flow. Mounted at the 45° angle, it has to move through an angle of only 45° to fully open, whereas if it were mounted perpendicular to the flow it would have to move through an angle twice as large. It is of extreme simplicity and low cost of materials and fabrication. Also no part of thevalve200 projects intopump chamber30. This minimizes the volume ofpump chamber30 which helps make the pump self-priming and increases its efficiency. Further contributing to these characteristics is that theflex valve200 is biased closed when the pump is not operating.

FIGS. 5 and 6 show alternative embodiments of the pump of the invention. The pump inFIG. 5 is essentially the same as that ofFIG. 2 except that thepump chamber30 is reduced in thickness to that of the sealingwasher34. This improves the self-priming ability of the pump. The pump inFIG. 6 also has a minimallythick pump chamber30. Further, theinlet22 andoutlet24 are perpendicular to the plane of theactuator14, a configuration that may be more convenient in some applications.

In yet another embodiment, not shown, the bottom of the pump body comprises apiezoelectric actuator14 arranged identically but as a mirror image of thepiezoelectric actuator14 just described, with the substrate layers40 facing each other across thepump chamber30.

In still another embodiment, not shown, two of the pumps above described are mounted side by side in one pump body. The actuator; seals; inlets and outlets, with one-way valve in the inlets only; pump covers; and drivers are positioned in one or more of the configurations described above. In a preferred form of this embodiment, the drivers are in series electrically, with the pumps operating in parallel fluidwise in the system in which they are deployed.

INDUSTRIAL APPLICABILITY

This invention has particular application for water cooling of the CPU in computers but may have wider applications wherever a very small pump of relatively high flow rate and minimum power consumption is needed to move liquids at very low cost. The piezoelectric actuator by itself can have very many other applications, such as speakers, audible alarms, automotive sensors, sound generators for active noise cancellation, and accelerometers.

Claims (20)

1. A method for making a piezoelectric actuator comprising:

(1) coating at least one of a first surface and a second surface of a piezoelectric element with a polyimide adhesive;

(2) heating the piezoelectric element to dry the adhesive; then

(3) inserting the piezoelectric element between a first metallic layer and a second metallic layer to form an assembly;

(4) placing the assembly in a press;

(5) while the assembly is in the press, curing the polyimide adhesive at a curing temperature which does not depole the piezoelectric element, thereby bonding the piezoelectric element between the first metallic layer and the second metallic layer.

2. The method ofclaim 1, wherein step (1) comprises coating the piezoelectric with a thin layer of polyimide adhesive no more than 0.005 inch thick.

3. The method ofclaim 1, further comprising, as act (2) using heat to initiate a polymerization process.

4. The method ofclaim 1, wherein the polyimide adhesive is a polyimide gel adhesive, and further comprising, as act (2) using heat to remove solvent from the polyimide gel adhesive.

5. The method ofclaim 1, further comprising performing act (3) only after the polyimide adhesive is dry to the touch.

6. The method ofclaim 1, further comprising, at least during act (5), applying uniform pressure to the piezoelectric actuator formed by bonding the piezoelectric element between the first metallic layer and the second metallic layer.

7. The method ofclaim 6, wherein the act of applying pressure comprises:

(a) laying a polyimide film on a bottom plate of a press;

(b) placing the piezoelectric actuator on the polyimide film;

(c) covering the piezoelectric actuator with a sheet of rubber;

(d) tightening the press whereby the bottom plate and a top plate of the press apply uniform pressure to the piezoelectric situated therebetween.

8. The method ofclaim 6, further comprising:

(e) cooling the piezoelectric actuator in the press;

(f) removing the piezoelectric actuator from the press.

9. The method ofclaim 1, wherein act (5) comprises inserting the assembly in the press, and then placing the press in a heater.

10. The method ofclaim 1, further comprising using a polycrystalline material as the piezoelectric element.

11. The method ofclaim 1, further comprising using stainless steel as the first metallic layer.

12. The method ofclaim 1, further comprising using aluminum as the second metallic layer.

13. The method ofclaim 1, further comprising forming the first metallic layer with a rim arranged to serve as a clamping surface.

14. The method ofclaim 13, further comprising forming the first metallic layer to have a larger surface area than the piezoelectric element.

15. The method ofclaim 14, further comprising forming the second metallic layer to have a smaller surface area than the piezoelectric element.

16. The method ofclaim 1, further comprising:

forming the first metallic layer to have a larger diameter than the piezoelectric element; and

forming the second metallic layer to have a smaller diameter than the piezoelectric element.

17. The method ofclaim 1, further comprising using a flat element as the piezoelectric element.

18. The method ofclaim 1, further comprising using as the polyimide adhesive as a gel which has a minimum of 25% solids.

19. The method ofclaim 1, wherein the curing temperature does not melt the polyimide adhesive.

20. The method ofclaim 1, wherein the curing temperature is no more than 200 degrees Centigrade.

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